Have you checked for bedbugs or mosquitos?
My son has had these bumps for a few months now, i was told by his pedi that if they were not bothering him and they were not red or with pus then I should just wash it with mild soap and pat it dry with a towel, I've been doing just that and they have gone down although they are still visible but do not bother him in any way :)
Have they all had their immunizations? Is it itchy? Do they have a fever? Hives, chicken pox, measles all need to be ruled out.